To reliably transmit binary digital data between a source point and a destination point, it is necessary to provide the destination with a means of determining when the data has changed state. When the two states in a binary digit scheme are represented by different voltage levels, typically the destination compares the received signal to a reference voltage. Usually, this reference voltage is implicit as it is derived from a common ground connection. However, in cases where the ground is not common, or where there are significant DC offsets in the ground, it is necessary for the source to transmit a reference level to the destination. This may also be necessary where AC noise is present on the ground and power planes. Typically, to transmit a reference signal from the source to the destination, a differential scheme is used. In a differential scheme, the binary complement of the signal is sent along with the signal. At the destination the signal can be compared to its complement and the two states resolved.
It is well known to use differential line drivers and differential receivers in high-speed data busses. The connection between drivers and receivers can be provided by twisted pairs that are in cable or ribbon form. Flat ribbon cable may be used in single-ended fashion with every other conductor grounded at both the driver and the receiver end. The above is described in "MECL System Design Handbook" 4th Ed., William R. Blood, Jr., Motorola Semiconductor Products, Inc., 1988, pp 70-76. While the noise immunity of differentially driven and received twisted pair cable is well known and very desirable, it comes at the expense of having to provide two lines for each data line in the bus.